GSM band at 900MHz will propagate further than GSM band at 1800MHz.
The higher the frequency ... the more loss through free space or objects.
for all the factors from the transmitter to the receiver:
Path loss equation formula
where: GTx = transmitter antenna gain
GRx = receiver antenna gain
λ = wavelength (same units as d): Freq=1/λ
d = distance separating Tx and Rx antennas
L = system loss factor (≥ 1)
In adition to last reply,
lower area is covered in 1800M frequency band rather than 900M.
In fact, every site(antenna) specializes 1800M band to more near
and non moving moblie stations.
Furthermore, the number of carriers in 1800M band is 374 and it is about
3 times the one in 900MHz (124 carriers).
The band 1800 has more loss, but has higher quality of speech.